Despite fabrication problems, .gamma.' strengthened nickel base superalloys are extensively employed in high performance environment articles. In general, the problems with such alloys relate to the high solvus temperature of the .gamma.' phase. This temperature usually has a value very close to the incipient melting point of the alloy.
Direct hot isostatic pressing of superalloy powder has been used to produce large components in a near-net shape in order to reduce material use and machining waste. However, current processing routes include blank die compaction followed by extrusion and forging.
Considerable effort has been expended in developing alloys and techniques for use in powder metallurgy and processing. Superalloy compositions providing improved resistance to fatigue crack growth at elevated temperatures are particularly useful in making shaped articles by various powder processes.
Crack propagation rate in high strength superalloy bodies depends on applied stress and crack length. These factors combine to provide a crack growth driving force referred to as stress intensity which is proportional to the applied stress multiplied by the square root of crack length K.apprxeq..sigma..sqroot..alpha..
The most undesirable time-dependent crack growth behavior occurs when a hold-time is imposed at peak stress during the cycle, i.e., when the maximum tensile stress is held constant for a period of time.
Various nickel base alloy compositions are representative of alloying situations in which many of the same elements are combined to achieve distinctly different functional relationships between the elements which provide the alloy system with different physical and mechanical characteristics. However, it is still not possible for workers in the art to predict with any degree of accuracy the physical and mechanical properties that will be displayed by certain concentrations of commonly known elements used in varying combinations to form such alloys even though such combinations may fall within broader more generalized teachings disclosed in the art. This inability to predict is particularly noticeable.
Certain objectives for forgeable nickel base superalloys are known. In the first place, it is important to minimize the time dependence of fatigue crack resistance and to secure values of strength at room and elevated temperature and creep properties that are reasonably comparable to those of known powder processed alloys. It is also important to reduce processing difficulties encountered in previous metal working procedures.